The cage calculator gives you a rough idea of how many rats can comfortably fit into a cage. This only shows you so you don't put 15 in a cage that's not big "enough" for 6. However, the Martins R-685 is great for 2 ratties. If you get the flip top added on (I believe you will need to ask for it) it will make cage cleaning tons easier.

Like Molucass said, 2 male rats can be introduced to eachother even if they haven't grown up together. While introducing either sex when they're young is easy, sometimes that just isn't possible. The older they get, the longer introductions can take. Some take weeks, and some take days. But, I do suggest going ahead and getting two from the same litter, or adopting two that have lived together all their lives. It just makes it a whole lot easier than you.

To answer one of your original questions, yes, rats can get neutered. Typically females are more expensive because it's internal, as opposed to external. Both still have their perks. Spaying can reduce the chance of getting mammary tumours among other things. Neutering can reduce chances of cancers that could possibly run in certain lines. And if you have a boy and girl living together, get at least one of them neutered, please.

With the food situation, I also agree to not give a store bought mix, unless it's a reputable brand. Still, even with that, I'd suggest Suebee's, or your version of Suebee's. The Suebee's Mix site tells you exactly what rats need in their diets, how much, and the easiest ways to get them their nutrients. Along with their daily mix, they'll need a staple of some kind. Either a lower protein/fat dog food, or a good lab block. You can order lab blocks from Kim's Ark here.

For my parting words, I second Molucass' recommendation on Ratpalace and RatsRule! Awesome pet rat sites that would love to have you as part of their community! Join us.